finding 18.5 : extreme-rainfall-and-flooding-impacts

Extreme rainfall events and flooding have increased during the last century, and these trends are expected to continue, causing erosion, declining water quality, and negative impacts on transportation, agriculture, human health, and infrastructure.



This finding is from chapter 18 of Climate Change Impacts in the United States: The Third National Climate Assessment.

Process for developing key messages: The assessment process for the Midwest Region began with a workshop was that was held July 25, 2011, in Ann Arbor, Michigan. Ten participants discussed the scope and authors for a foundational Technical Input Report (TIR) report entitled “Midwest Technical Input Report.”2626b5ca-ec04-4e41-8405-9f582c779a7a The report, which consisted of nearly 240 pages of text organized into 13 chapters, was assembled by 23 authors representing governmental agencies, non-governmental organizations (NGOs), tribes, and other entities. The Chapter Author Team engaged in multiple technical discussions via teleconferences that permitted a careful review of the foundational TIR2626b5ca-ec04-4e41-8405-9f582c779a7a and of approximately 45 additional technical inputs provided by the public, as well as the other published literature, and professional judgment. The Chapter Author Team convened teleconferences and exchanged extensive emails to define the scope of the chapter for their expert deliberation of input materials and to generate the chapter text and figures. Each expert drafted key messages, initial text and figure drafts and traceable accounts that pertained to their individual fields of expertise. These materials were then extensively discussed by the Author Team and were approved by the Chapter Team members.

Description of evidence base: The key message and supporting text summarize extensive evidence documented in the Technical Input Report.2626b5ca-ec04-4e41-8405-9f582c779a7a Technical inputs on a wide range of topics were also received and reviewed as part of the Federal Register Notice solicitation for public input. Evidence for extreme weather and increased precipitation across the U.S. are discussed in Chapter 2 (Our Changing Climate, Key Messages 5, 6, and 7) and its Traceable Accounts. Specific details for the Midwest are detailed in “Climate Trends and Scenarios for the U.S. National Climate Assessment”95f2ea7d-12e3-4ed5-9247-7cf139db91a9 with its references. A recent bookb228ac0d-7bf9-4391-99e7-5c598b9ce55e also contains chapters detailing the most current evidence for the region. There is compelling evidence that annual total precipitation has been increasing in the region, with wetter winters and springs, drier summers, an increase in extreme precipitation events, and changes in snowfall patterns. These observations are consistent with climate model projections. Both the observed trends and climate models suggest these trends will increase in the future. Recent records also indicate evidence of a number of high-impact flood events in the region. Heavy precipitation events cause increased kinetic energy of surface water and thus increase erosion. Heavy precipitation events in the historical records have been shown to be associated with discharge of partially or completely untreated sewage due to the volumes of water overwhelming combined sewer systems that are designed to capture both domestic sewage and stormwater. Climate downscaling projections tend to indicate an increase in the frequency and duration of extreme events (both heavy precipitation and meteorological drought) in the future. An extensive literature survey and synthetic analysis is presented in chapters in a recent bookb228ac0d-7bf9-4391-99e7-5c598b9ce55e for impacts on water quality, transportation, agriculture, health, and infrastructure.

New information and remaining uncertainties: Precipitation is much less readily measured or modeled than air temperature.6f71c947-69a3-4b13-af70-f061f2e6671f Thus both historical tendencies and projections for precipitation are inherently less certain than for temperature. Most regional climate models still have a positive bias in precipitation frequency but a negative bias in terms of precipitation amount in extreme events. Flood records are very heterogeneous and there is some ambiguity about the degree to which flooding is a result of atmospheric conditions.e15600d0-290f-44e2-9b58-9ffd295ee6d2 Flooding is not solely the result of incident precipitation but is also a complex function of the preceding conditions such as soil moisture content and extent of landscape infiltration. A key issue (uncertainty) is the future distribution of snowfall. Records indicate that snowfall is decreasing in the southern parts of the region, along with increasing lake effect snow. Climate models predict these trends will increase. There is insufficient knowledge about how this change in snowfall patterns will affect flooding and associated problems, but it is projected to affect the very large spring floods that typically cause the worst flooding in the region. In addition, recent data and climate predictions indicate drier summer conditions, which could tend to offset the effects of higher intensity summer storms by providing increased water storage in the soils. The relative effects of these offsetting trends need to be assessed. To determine future flooding risks, hydrologic modeling is needed that includes the effects of the increase in extreme events, changing snow patterns, and shifts in rainfall patterns. Adaptation measures to reduce soil erosion and combined sewer overflow (CSO) events are available and could be widely adopted. The impacts of increased magnitude of heavy precipitation events on water quality, agriculture, human health, transportation, and infrastructure will be strongly determined by the degree to which the resilience of such systems is enhanced (for example, some cities are already implementing enhanced water removal systems).

Assessment of confidence based on evidence: There have been improvements in agreement between observed precipitation patterns and model simulations. Also an increase in extreme precipitation events is consistent with first-order reasoning and increased atmospheric water burdens due to increased air temperature. Recent data suggest an increase in flooding in the region but there is uncertainty about how changing snow patterns will affect flood events in the future. Thus there is high confidence in increases in high-magnitude rainfall events and extreme precipitation events, and that these trends are expected to continue. There is medium confidence that, in the absence of substantial adaptation actions, the enhancement in extreme precipitation and other tendencies in land use and land cover result in a projected increase in flooding. There is medium confidence that, in the absence of major adaptation actions, the enhancement in extreme precipitation will tend to increase the risk of erosion, declines in water quality, and negative impacts on transportation, agriculture, human health, and infrastructure.6f71c947-69a3-4b13-af70-f061f2e6671f

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